1. Field of the Invention
The present invention relates to a charged particle beam apparatus, a charged particle detection method, and a method of manufacturing a semiconductor device.
2. Related Background Art
Various methods have been proposed for increasing the efficiency of detection of secondary electrons generated from the surface of a specimen in a scanning electron microscope, such as a method of disposing a micro channel plate (MCP) detector and a metal casing to which a negative potential is applied, in a lower portion of the objective lens (such as that disclosed in Japanese Patent Laid-Open No. 63-175325) and a method of disposing an MCP detector in an upper portion of the objective lens (such as that disclosed in Japanese Patent Laid-Open No. 06-243814 and Japanese Patent Laid-Open No. 11-162384). Note that the terminology is used in this document so that “charged particles” has a broad meaning that includes reflected charged particles and “secondary electrons” has a broad meaning that includes reflected electrons, as long as they are not particularly mentioned otherwise.
The charged particle detection device proposed in Japanese Patent Laid-Open No. 63-175325 is provided with an MCP detector to detect secondary electrons and a metal casing in a lower portion of the objective lens that focuses the electron beam onto the wafer W to irradiate it. Secondary electrons that are generated from a position on the wafer that has been irradiated by the electron beam are drawn in the direction of the optical axis by the magnetic field of the objective lens 65, but these secondary electrons are repelled by the electrical field generated by the negative potential (of −50 to −150 V) that is applied from a power source to the metal casing, to follow a trajectory that detours even further below the lower edge of the metal casing and be detected by the lower surface of the MCP detector.
The scanning electron microscope disclosed in Japanese Patent Laid-Open No. 06-243814 comprises two electron detectors disposed so as to sandwich the objective lens, making it possible to detect secondary electrons (where “secondary electrons” is used in the narrower sense, excluding reflected electrons) and reflected electrons simultaneously, by detecting the secondary electrons (in the narrower sense) emitted from the wafer by the electron detector on the scan deflector side and detecting the reflected electrons emitted from the wafer by the electron detector on the wafer side.
In the scanning electron microscope disclosed in Japanese Patent Laid-Open No. 11-162384, the secondary electrons emitted from the wafer are accelerated between the wafer and the objective lens immediately after emission by an electrical field that draws them out on the electron gun side, in a state in which a negative potential (a retarding potential) is applied to the wafer, making it difficult to detect them at a lower portion of the objective lens. For that reason, a Wien filter is provided between the objective lens and the MCP detector disposed on the upstream side thereof, to excite an orthogonal electromagnetic field by this Wien filter. The deflection force with respect to the primary electron beam is canceled by this orthogonal electromagnetic field, but the deflection force with respect to the secondary electrons generated from the wafer and traveling through the objective lens is not canceled thereby so that the secondary electrons are deflected away from the optical axis and thus can be detected by the MCP detector that is disposed around the perimeter of the optical axis.
However, the above-described detection methods have problems, as discussed below.
Specifically, the effect of the method disclosed in Japanese Patent Laid-Open No. 63-175325 only occurs with an optical system in which the distance from the objective lens to the wafer is sufficiently long and the negative potential applied to the metal casing is comparatively large (−50 to −150 V), which lead to blurriness of the focus.
If an electron beam is accelerated to a high acceleration and strikes a resist on the wafer, the proximity effect occurs, in which the irradiating electron beam is reflected by the various thin films formed in the lower surface of the resist on the upper surface of the wafer and is again directed upward from the resist. Since this causes deterioration in the focus and resolution of the drawing pattern, a charged particle beam lithography apparatus employing a shaping aperture method using an electron beam with a low-acceleration voltage has been proposed recently. In this low-acceleration voltage type of charged particle beam lithography apparatus, loss of focus of the beam due to the space charge effect becomes a problem, and thus it is essential to reduce the dimensions of the optical system in order to reduce this loss of focus. With the detection methods disclosed in Japanese Patent Laid-Open No. 06-243814 and Japanese Patent Laid-Open No. 11-162384, the MCP detector is disposed above the objective lens, making it necessary to provide sufficient distance between the wafer and the MCP detector in both methods. For that reason, these methods cannot be applied to a low-acceleration voltage type of charged particle beam lithography apparatus in which the optical system must be compact.